1. Field of the invention
The present invention relates to an upper mounting structure for a wheel suspension, especially for a strut-type wheel suspension, which may be suitably used for suspending a wheel from a vehicle body, and in particular relates to an upper mounting structure for a wheel suspension, which is capable of reducing mainly transverse vibration of the vehicle body due to a surge vibration of a suspension coil spring, thereby providing a comfortable ride feeling of a vehicle.
2. Description of the Related Art
A strut-type wheel suspension usually consists of a strut assembly, a suspension coil spring, a lower mounting structure and an upper mounting structure. Typical example of a conventional upper mounting structure for a strut-type wheel suspension is as shown in FIG. 1. The shown structure in FIG. 1 is of a so-called isolated input type and is disclosed, for example, in Japanese Utility Model Application Laid-Open Publication No. 62-25206.
In such upper mounting structure for a strut-type wheel suspension a suspension coil spring 1 is seated at its upper end near a vehicle body on a spring support 2 which is resiliently secured to an upper support 4 through a spring insulator 3 made from solid rubber material. The upper support 4 is adapted to be fastened to the vehicle body not shown by a bolt and nut means. In addition, the coil spring 1 is resiliently or rigidly engaged at its lower end with an outer sleeve not shown of a strut assembly 5 which further includes a piston rod 5a telescopically movable with respect to the above-mentioned outer sleeve.
The piston rod 5a penetrates freely through a central opening of the upper support 4 and is resiliently secured thereto through a pair of strut mounting insulators 6 made from solid rubber material. These strut mounting insulators 6 are arranged on either side of the upper support 4 to hold it therebetween and to make sure a connection between the piston rod 5a and the upper support 4.
Although not shown, the outer sleeve of the strut assembly 5 is resiliently or rigidly coupled with a knuckle not shown which is articulated to the vehicle body or a subframe fixed thereto as a part of the vehicle body through a plurality of links. This structure comprises the lower mounting structure for the strut-type wheel suspension. Alternatively, the lower mounting structure for the strut-type wheel suspension may be composed of the outer sleeve of the strut assembly, which is resiliently or rigidly coupled with a hub carrier articulated to the vehicle body or the subframe fixed thereto as a part of the vehicle body through a plurality of links. In FIG. 1, moreover, 7 represents a bumper rubber and 8 represents a dust boot respectively.
FIG. 2 shows a vibration system model of the upper mounting structure in FIG. 1. As appreciated from FIG. 2, in the structure shown in FIG. 1 the coil spring 1 and the spring insulator 3 is arranged in serial relation with each other so as to form one vibration route, while the strut assembly 5 and the rod mounting insulator 6 is arranged in serial relation with each other so as to form the other vibration route. These vibration routes exist in parallel relation with each other between the upper support 4 (the vehicle body) and the wheel 9 to be suspended therefrom and are isolated from each other.
The conventional upper mounting structure of an isolated input type as mentioned above with respect to FIGS. 1 and 2 has following problems which will be discussed. In the isolated input type structure, the strut mounting insulator 6 is preferably designed to have relatively low stiffness (low spring constant) in vertical, longitudinal and transverse directions of the vehicle body in order to prevent low frequency vibration from a road to the strut assembly 5 from transmitting to the vehicle body and thus to realize a comfortable ride feeling of the vehicle. On the contrary, the spring insulator 3 has to support vehicle weight only by itself and then is preferably designed to have higher stiffness (higher spring constant) than the strut mounting insulator 6 (for example, more than twice of stiffness of the insulator 6). These designs of insulators 3 and 6 give rise to a coexistence of the comfortable ride feeling and the reliable support of vehicle weight at high level.
The conventional upper mounting structure of an isolated input type, however, is not capable of enhancing a performance which suppresses a transmission of vibration to the vehicle body, as will be appreciated from following descriptions.
In FIGS. 1 and 2, when a vertical surge vibration of the coil spring 1 is energized by vibration input from a road, this surge vibration is also transmitted to the outer sleeve of the strut assembly 5 through a lower end of the coil spring 1, since the spring insulator 3 is of high stiffness as mentioned above. On the other hand, within a surge vibration range of high frequency at which the coil spring 1 tends to cause said surge vibration, a telescopical movement between the piston rod 5a and the outer sleeve of the strut assembly 5 is remarkably suppressed because of a large damping effect thereof within the higher frequency range. Therefore, the surge vibration to the strut assembly 5 from the coil spring 1 is directly transmitted to the piston rod 5a of the strut assembly 5. Still now, the strut mounting insulator 6 is of a relatively low stiffness as mentioned above, and thus the strut assembly 5, upon acceptance of the surge vibration, is easily oscillated in the axial direction as a whole. This axial oscillation of the strut assembly 5 is transmitted to the vehicle body as follows.
The axial oscillation of the strut assembly 5 causes the knuckle to be rocked about a center of gravity thereof and this rock motion of the knuckle results in transverse vibration of the vehicle body since the knuckle is articulated to the vehicle body through the plurality of links. This transverse vibration of the vehicle body also gives rise to a compartment noise.